The present invention relates to a method and an apparatus for color correction when a color image is formed, and in particular to a method and an apparatus for color correction suitable for application on a color printer, which is used to form color image on a direct heat-sensitive color recording medium where cyan heat-sensitive color developing layer, a magenta heat-sensitive color developing layer, and a yellow heat-sensitive color developing layer are sequentially deposited on each other.
The present applicant previously proposed a direct heat-sensitive color recording medium as shown in FIG. 7. This medium can be briefly described as follows: In FIG. 7, on a paper base 1, a cyan heat-sensitive color developing layer 2, a magenta heat-sensitive color developing layer 3, and a yellow heat-sensitive color developing layer 4 are deposited sequentially on each other, and a heat-resistant protective layer 5 is provided on the uppermost layer. In the following, cyan is abbreviated as "C" magenta as "M", and yellow as "Y".
The C heat-sensitive color developing layer 2 comprises a micro-capsule enveloping a diazonium chloride compound having high reaction activity and a C coupler. The M heat-sensitive color developing layer 3 comprises a micro-capsule enveloping a diazonium chloride compound with high reaction activity and an M coupler. The Y heat-sensitive color developing layer 4 comprises a micro-capsule enveloping a diazonium chloride compound having high reaction activity and a Y coupler.
Each of the C heat-sensitive color developing layer 2, the A heat-sensitive color developing layer 3, and the Y heat-sensitive color developing layer 4 has heat-sensitive property shown in FIG. 9, and optical density, i.e. coloring density, is determined according to the value of the heat energy given.
Next, description will be given on a method to form a color image on a direct heat-sensitive color recording medium referring to FIG. 8. In FIG. 8, the protective layer 5 is not illustrated.
As shown in FIG. 8(a), color is developed by the Y coupler in the Y heat-sensitive color developing layer by heating with low energy using a thermal head 6. Next, as shown in FIG. 8(b), ultraviolet ray with a first wavelength is emitted from a lamp 7. Thus, on diazo compound not yet developing color in the Y heat-sensitive color developing layer 4, photo dissociation is performed, and it is fixed. As a result, the Y coupler will not develop color any more even when it may be heated. An image of Y component is thus formed.
Next, an image of M component is formed. In this case, also, as shown in FIG. 8(c), color is developed by the H coupler in the M heat-sensitive color developing layer 3 by heating with moderate energy using the thermal head 6. Then, as shown in FIG. 8(d), ultraviolet ray with a second wavelength is emitted from the lamp 7, and on diazo compound not yet developing color in the M heat-sensitive color developing layer 3, photodissociation is performed, and it is fixed. As a result, the M coupler will not develop color any more even when it may be heated.
Next, an image of C component is formed. In this case, as shown in FIG. 8(e), color is developed by the C coupler in the C heat-sensitive color developing layer 2 by heating with high energy using the thermal head 6. For the C color, fixation may be carried out using ultraviolet ray of a predetermined wavelength, while it is known that there is practically no problem even when fixation is not performed.
By the process described above, an image can be formed in full color.
However, when an image is formed in full color on a direct heat-sensitive color recording medium as described above, color mixing may occur. Specifically, as it is evident from FIG. 9, when heating is performed with energy in the range of E.sub.1 -E.sub.2, color develops not only by Y but also by M. When an image is to be formed with Y component only and the Y coupler is heated with energy of E.sub.1 -E.sub.2 to develop color in high density, not only Y but also M is developed a little, and color mixing occurs. Similarly, when it is heated with energy of E.sub.3 -E.sub.4 in FIG. 9, not only M but also C is developed a little. When it is wanted to form an image of M component only and it is tried to develop M in high color density, not only M but also C is developed a little when heating with energy of E.sub.3 -E.sub.4, and color mixing occurs.
The color mixing as described above also occurs in the cases given below in addition to the above two cases: One is the case where it is wanted to form a red image including M in high density and it is heated with energy of E.sub.3 -E.sub.4 in FIG. 9 when the image of M component is formed. In this case, C is also developed a little and color mixing occurs, and the reproducibility of the developed red color may not be satisfactory.
The other is the case where it is wanted to form a green image including Y in high density and it is heated with energy of E.sub.1 -E.sub.2 of FIG. 9 when the image of Y component is formed. In this case, M is also developed a little and color mixing occurs, and the reproducibility of the obtained green color may not be satisfactory.
No problem of color mixing occurs in the case other than the above. For example, in case an image in black color in high density is formed, even when M is developed during the formation of an image of Y component, there is no problem because M is then developed in high density during the formation of the image of M component. Even when C is developed during the formation of the image of M component, there is no problem because C is developed in high density when the image of C component is formed.
In case a red image is formed, the image of Y component is formed at first. In this case, even when M is developed, no problem of color mixing occurs because M is developed in the process to form the image of M component.
Further, the problem of color mixing does not occur in case a blue image including M in high density is formed. In this case, the image of M component is formed by heating with energy of E.sub.3 -E.sub.4 of FIG. 9. In this case, even when C is developed, the problem of color mixing does not occur because C is developed in the next process to form the image of C component.
Additional description is given now for reference. As it is evident from the description on the process to form color image in the above, the image of M component is formed always after fixation of Y color. For example, in case an image of M component is formed during the formation of an image of M component only, fixation of Y color is carried out prior to the formation of the image of M component. Even when it is heated with energy of E.sub.1 -E.sub.2 of FIG. 9 during the formation of the image of M component, Y is not developed. Similarly, during the formation of the image of C component only, Y color and M color are already fixed. Then, even when it is heated with energy of E.sub.3 -E.sub.4 in FIG. 9 during the formation of the image of C component, M is not developed.
Accordingly, when an image in full color is formed on a direct heat-sensitive color recording medium, the problem of color mixing arises in the following four cases:
(1) When an image of Y component only is formed and Y is developed in high density.
(2) When an image of M component only is formed and M is developed in high density.
(3) When an image in red color is formed and M is developed in high density.
(4) When an image in green color is formed and Y is developed in high density.
In the meantime, various types of color processing are performed on a color printer, and color mixing as described above may occur due to the technique of color processing. For example, chroma intensifying processing is sometimes performed by matrix calculation using a matrix of 3.times.3 to signals of R, G and B as given in the following equation: ##EQU1##
In the equation (1), matrix calculation is performed to the signals of R, G and B, but it is needless to say that it can also be carried out to the signal of Y, M and C.
When the matrix calculation is performed as described above, characteristics as shown in FIG. 10 can be obtained. FIG. 10 is a diagram to show the "signal value to density" characteristics in M. In this figure, the characteristics given by A are the characteristics of M when an image of M component only is formed and when an image of M component is formed during the formation of a red image. The characteristics shown by b represents the characteristics of M when an image of M component is formed during the formation of a gray image. Naturally, similar characteristics are described for Y and C.
Specifically, when a gray image is formed, the characteristics of M are the one shown by b in FIG. 10. When, an image of M only and a red image are formed, the density of M is higher than the density during the formation of the gray image as shown by a in FIG. 10, i.e. high contrast. Thus, chroma is intensified.
The chroma intensifying processing as described above is generally effective. When such chroma intensifying processing is performed during the formation of image on the direct heat-sensitive color recording medium, heating with high energy is carried out even to M which has moderate signal value. As a result, color mixing as described above may be more likely to occur.
To avoid the color mixing as described above, a method may be used, in which Y and M are not used in high density. That is, in case Y is developed, it is heated with energy lower than E.sub.1 in FIG. 9. In case M is developed, it is heated with energy lower than E.sub.3 in FIG. 9. It is evident that the problem of color mixing does not occur if Y and M are not used in high density.
However, if Y and M in higher density are not used, it is impossible to obtain pure color when M is mixed with C although M may be mixed with Y. Black color in high density cannot be developed.